The "Run Claude Code Action" step maps the auth inputs into env
unconditionally:
ANTHROPIC_API_KEY: ${{ inputs.anthropic_api_key }}
CLAUDE_CODE_OAUTH_TOKEN: ${{ inputs.claude_code_oauth_token }}
When the input is empty — whether because the caller supplied auth via
the step `env:` block (as reported in #676) or because the `with:` value
resolves empty in some runner/secret configurations — this assignment
overwrites the inherited env value with an empty string. validate-env
then fails with the misleading "Either ANTHROPIC_API_KEY or
CLAUDE_CODE_OAUTH_TOKEN is required" error even though the caller did
provide a token.
Fall back to the inherited env var when the input is empty, mirroring the
existing `${{ env.X }}` pattern already used a few lines below for
ANTHROPIC_BASE_URL / ANTHROPIC_CUSTOM_HEADERS. The input still takes
precedence; nothing changes for workflows that pass auth via `with:`.
Fixes#676
